It wasn't exactly common knowledge, but was generally understood by those who knew him, that Severus Snape did not like dogs.
It was only to himself, and now, to you, that Severus would admit that he was terrified of dogs.
Severus certainly had a lot of rational fears; he had seen and done so many horrifying things in his life, and there were things yet to do which he dreaded. But if he were ever confronted by a boggart, it would probably choose this less rational fear to transform into. It was a base dread. Child-like. It was a fear of sharp things and loud things and big, dangerous, monstrous things. You had assumed his fear was reserved for larger dogs. Admittedly wolf-like dogs. And you could understand that, after all he'd been subjected to a lifetime ago.
When you had revealed to him that you were an animagus, he'd been begrudgingly impressed with your transfigurative prowess. It certainly wasn't easy magic to perform, and he had to give you credit for that, but it was clear that his views on animagi were tainted by past experiences. And this opinion did not improve when you further revealed that your form was that of a dog. In fact, any fascination that he might have held for the subject seemed to drop entirely from his already pale face when you mentioned it. Which had led to the difficult conversation about his fears.
Which had ultimately led to the present moment, with Severus sitting with his legs pulled up on his threadbare couch in Spinners End, watching anxiously as you bumbled toward him as a small, spotted dog with a rapidly wagging tail. You placed your over-large paws on the edge of the couch, releasing a happy yap, which caused Severus to wince. Your wagging tail slowed at that, and Severus withered guiltily.
"I didn't sign up for immersion therapy, Katelynn," he murmured, looking apprehensive as he reached out a hand, patting the space beside him. His body seemed to tense as you bounced up onto the couch, thought you made sure to keep a fair distance. You'd barely managed to talk him into letting you show him; you certainly didn't wish to scare him off. You simply sat, tail thumping against the cushions as you watched one another. Severus swallowed a hard lump in his throat, before holding his hand out again. He didn't move to pet you, so you made the move for him, licking his palm before bumping your head against it. He made a soft noise, something like a restrained snort, as he curled his fingers against your soft ears.
"You're lucky you're cute," he sighed, and you couldn't stop your tail from speeding up if you tried.